1. Field of the Invention
This invention relates to rotor structures of dynamoelectric machines, such as turbine generators, and in particular, to means for shunting currents induced in rotor surfaces by externally applied magnetic fields away from retaining rings situated at both ends of the rotor.
2. Description of the Prior Art
Generator rotors according to prior art include shaft and body portions of relatively small and large diameter, respectively. Longitudinal slots usually extend axially along the outer periphery of the rotor body. Longitudinal electrical conductor portions disposed in different rotor slots are normally interconnected at each axial end of the rotor body by end turn electrical conductor portions. The longitudinal conductor portions are radially restrained by wedges inserted in those slots radially outside the longitudinal electrical conductors. The end turn portions, on the other hand, are radially restrained during high speed rotor rotation by retaining ring structures which usually constitute high strength cylindrical members which are normally affixed to the rotor body's outer periphery through a shrink fitting process. Since the end turn portions lie radially beneath the retaining rings, centrifugal forces exerted on those end turn portions during rotor rotation are counteracted by internal forces in the retaining rings.
During normal service, central station turbine generators operate with substantially balanced polyphase (commonly three phase) loads. Generator operation is, however, sometimes required for unbalanced load whose duration may be continuous or short. An example of the latter is an unbalanced fault typically occurring as a line-to-line short circuit. During unbalanced load operation or unbalanced fault occurrence, stator windings of turbine generators or other dynamoelectric machines carry a system of currents, denoted in technical parlance as a negative-sequence system, which generates magnetomotive forces that rotate at the generator's synchronous speed in a direction opposite that of rotor rotation. Such magnetomotive forces induce currents in the conducting paths of the rotor (primarily at the surface) and at twice the line frequency. For example, such rotor induced currents for a 60 hertz generator have a frequency of 120 hertz. Analyzing the paths in which these induced surface currents flow in the complexly shaped, discontinuous rotor structure is a difficult task and has been the subject of much design effort.
The induced rotor surface currents travel in a substantially axial direction and eventually enter the rotor's retaining rings. High strength alloys which are customarily utilized for retaining rings usually have relatively high electrical resistance which causes substantial I.sup.2 R heating losses. Additionally, since each retaining ring is normally joined (typically shrunk on) to the rotor body in its restraining configuration, heating losses therein tend to expand the retaining ring and reduce the shrink fit pressure between it and the rotor body. As such, the electrical connection between the retaining ring and the rotor body develops a higher resistance which leads to higher heating losses and further aggravates the undesirable situation. For a given load unbalance or fault, dynamoelectric machines of higher rating will usually sustain higher heating losses in the retaining rings and induce greater thermal expansion thereof.
Previous attempts by machine designers to minimize current flow in the retaining rings include the use of end amortisseur connecting bars or rings which are highly conductive damper members operationally disposed radially inside the retaining rings and radially outside the end turn portions. Use of such highly conductive dampers was thought to shunt the induced rotor surface currents away from the retaining rings and thus reduce their heating and minimize disadvantages resulting therefrom. However, recent studies indicate that use of such radially inner dampers may provide very little relief from the I.sup.2 R heating affects of the induced rotor surface current on the retaining rings. Additional attempts to shield the retaining rings from induced rotor surface currents include U.S. Pat. No. 3,324,324 which issued June 6, 1967. Such structure shows insulation being interposed between the retaining rings and current carrying portions of the rotor body. Such structure, while reducing retaining ring heating, shunts the rotor surface current to other paths which are also vulnerable to overheating.